The present invention relates to a photodetector module for the direct optical coupling of an optical fiber and a photodetector.
The output light beam from an optical fiber is focused with a lens on the light receiving surface of a photodetector to couple the optical fiber and the photodetector. When a tapered optical fiber with a spherical tip is used for transmitting a light beam, the output light beam from the tapered fiber is directed directly toward the light receiving surface of a photodetector with the spherical tip of the tapered optical fiber to couple the optical fiber and the photodetector without using any lens. Since the former optical coupling method focuses the output light beam from the optical fiber with the lens, the diameter of the light receiving surface of the photodetector may be comparatively small. Therefore, wideband low-noise characteristics can readily be secured. However, such an optical coupling method needs a comparatively large photodetector module of a complicated construction.
Recently, a direct-coupling photodetector module employing a tapered optical fiber with a spherical tip has been proposed. This direct-coupling photodetector module is not provided with any lens and focuses the output light beam from the tapered optical fiber on the light receiving surface of the photodetector with the spherical tip of the tapered optical fiber. Since the spherical tip of the tapered optical fiber functions for focusing the output light beam on the light receiving surface of the photodetector, this photodetector module, as well as the photodetector module employing a lens for focusing, is capable of securing wideband low-noise characteristics.
A typical conventional direct-coupling photodetector module will be described with reference to FIG. 1. A ceramic submount 2 is mounted on a Kovar stem 1, and a photodetector chip 3, such as a photodiode (PD) chip, is mounted on the submount 2. A cap 4 formed of Kovar or a stainless steel is joined to the stem 1 by projection welding.
After assembling the photodetector chip 3, the cap 4 and other components on the step 1, a tapered optical fiber 5 with a spherical tip is disposed opposite to the photodetector chip 3, the position of a ferrule 6 relative to a sleeve 7 is adjusted as a light beam is projected through the tapered optical fiber 5 so that the optical axis of the tapered optical fiber 5 is aligned with the Z-axis, and the cap 4 and the sleeve 7 are moved relative to the cap 4 for adjustment with respect to the X-axis and the Y-axis to thereby attain an optimum optical coupling. Then, the sleeve 7 and the ferrule 6 are soldered respectively to the cap 4 and the sleeve 7 to seal the assembly hermetically. The photodetector module shown in FIG. 1 is provided with terminals 8 and a bonding wire 9. This conventional photodetector module requires difficult work for assembling because the position of the optical fiber 5 relative to the photodetector chip must be adjusted with respect the three axes, i.e., the X-axis, Y-axis and Z-axis.